Electron tube 6Z31 Tesla is a duodiode with undirectly heated oxide cathode, originally designed for rectifiers in the receivers. It is direct replacement for the 6X4 tube and after mechanical adjustments it may be used instead of 6C4P (6Ц4П) and EZ80 as well. Tubes are new, unused and in original boxes. Manufactured in the end of the 50's. Tested on BM215A.